CoQ10 is generally well tolerated, and no serious side effects have been reported in research. The most common side effects are mild: digestive upset, nausea, headache and trouble sleeping. The bigger safety question is not side effects but interactions, especially with the blood thinner warfarin, diabetes medicines and some cancer treatments. This guide covers what to expect, how much is considered safe, who should check with a GP or pharmacist first, and how to reduce the chance of side effects if you take a CoQ10 supplement.
The quick answer
- Usually mild: stomach upset, nausea, diarrhoea, loss of appetite, headache and insomnia are the side effects most often reported.
- Interactions matter more: CoQ10 may interact with warfarin and insulin, and may not suit people having chemotherapy or radiotherapy.
- Safe amounts: a 2006 safety review set an observed safe level of 1,200mg a day. Our serving is 600mg (2 capsules).
- Take it with food and split larger amounts across the day if your stomach is sensitive.
Is CoQ10 safe?
For most healthy adults, CoQ10 appears to be safe at the amounts used in supplements. CoQ10 (coenzyme Q10) is a compound your body makes itself and uses in normal cellular energy production, and small amounts come from foods such as meat, oily fish and wholegrains. The US National Center for Complementary and Integrative Health (NCCIH) states that no serious side effects of CoQ10 have been reported, and that mild side effects such as insomnia or digestive upset may occur.
That does not mean it suits everyone. CoQ10 can affect how some medicines work, and the research behind many of the uses people take it for is mixed. If you are wondering whether it is worth taking at all, our guide to what CoQ10 is good for explains what the evidence does and does not show.
Common CoQ10 side effects
Side effects are uncommon and usually mild. When they happen, they tend to be digestive and are more likely at higher amounts taken in one go.
| Side effect | How common | What helps |
|---|---|---|
| Stomach upset, nausea, diarrhoea, loss of appetite | The most often reported | Take it with a meal and split larger amounts across the day. |
| Trouble sleeping | Occasionally reported | Take it with breakfast or lunch rather than in the evening. |
| Headache, dizziness, tiredness | Occasionally reported | Try a lower amount. Stop and speak to a pharmacist if it continues. |
| Skin rash or itching | Rare | Stop taking it. Get urgent help if you have swelling of the face or trouble breathing. |
In a 2006 safety review of controlled trials, Hathcock and Shao found that digestive symptoms were the main effects reported, and that they did not rise in a clear way with dose (Hathcock and Shao, 2006).
Medicines CoQ10 may interact with
This is the most important safety section. The NCCIH notes that CoQ10 may interact with the blood thinner warfarin and with insulin, and that it may not be compatible with some types of cancer treatment. Always tell your GP, pharmacist or hospital team that you take CoQ10.
| If you take | Why it matters | What to do |
|---|---|---|
| Warfarin | CoQ10 is similar in structure to vitamin K and may make warfarin less effective. | Do not start CoQ10 without speaking to your anticoagulation clinic or GP. Your INR may need checking. |
| Insulin or other diabetes medicines | CoQ10 may affect blood sugar levels. | Speak to your diabetes team first and monitor your levels. |
| Chemotherapy or radiotherapy | The NCCIH notes CoQ10 may not be compatible with some cancer treatments. | Only take it if your oncology team agrees. |
| Blood pressure medicines | The evidence on CoQ10 and blood pressure is limited, but it is sensible to check. | Ask your pharmacist, and keep up any home blood pressure checks. |
| Statins | No harmful interaction is known. Statins lower the body's CoQ10 levels. | Never stop or change a statin to take CoQ10. See our CoQ10 and statins guide. |
How much CoQ10 is safe?
There is no official UK upper limit for CoQ10. The best-known safety benchmark comes from the 2006 risk assessment by Hathcock and Shao, which reviewed human trials and set an observed safe level of 1,200mg a day. Higher amounts have been used in some trials, but there is less safety data above that level.
Our CoQ10 300mg capsules contain ubiquinone, and the serving on the label is 2 capsules a day, which is 600mg. That is half the observed safe level. Do not exceed the stated serving, and if you take medicines, ask your GP or pharmacist whether this amount is right for you.
Many studies use lower amounts, often between 100mg and 300mg a day. If you are new to CoQ10 or have a sensitive stomach, starting with one capsule for a week or two before moving to the full serving is a sensible approach.
CoQ10, liver, blood pressure and blood sugar
Does CoQ10 affect the liver?
Liver problems are not a recognised side effect of CoQ10 at normal supplement amounts, and the 2006 safety review did not identify liver harm. If you have liver disease, speak to your GP or specialist before taking any supplement, because the liver processes most of what you take.
Can CoQ10 lower blood pressure?
The NCCIH notes that the small amount of evidence available suggests CoQ10 probably does not have a meaningful effect on blood pressure. Even so, if you take blood pressure medicines, mention CoQ10 to your pharmacist.
Can CoQ10 affect blood sugar?
The NCCIH lists a possible interaction with insulin. If you have diabetes, speak to your diabetes team before starting CoQ10 and keep monitoring your levels as usual.
CoQ10 side effects for women, pregnancy and breastfeeding
The side effects of CoQ10 are the same for women and men. What differs is pregnancy and breastfeeding: there is not enough safety research on CoQ10 supplements at these times, so speak to your GP or midwife before taking it. If you are taking CoQ10 while trying to conceive, for example on the advice of a fertility clinic, follow the clinic's advice on the amount and stop or check again once you are pregnant.
How to reduce the chance of side effects
- Take it with food. CoQ10 is fat-soluble, so taking it with a meal that contains some fat is the usual advice, and food also helps with stomach upset.
- Split the serving. If you take 2 capsules, try one with breakfast and one with lunch.
- Take it earlier in the day if you notice any effect on your sleep.
- Start low. Begin with one capsule a day and build up.
- Stick to the label. More is not better, and there is no benefit in going above the stated serving.
If you are choosing between forms, our guide to CoQ10 vs ubiquinol explains the difference. Side effects are similar for both.
Who should speak to a GP or pharmacist first
Check with a healthcare professional before taking CoQ10 if you:
- take warfarin or another blood thinner
- have diabetes or take insulin or other diabetes medicines
- are having chemotherapy or radiotherapy
- take blood pressure medicines or statins
- have liver or kidney disease, or a heart condition
- are pregnant, trying to conceive or breastfeeding
- are planning surgery, so your team knows everything you take
- want to give CoQ10 to someone under 18
The bottom line on CoQ10 side effects
CoQ10 is generally well tolerated, and the side effects that do occur are usually mild and digestive. Taking it with food, splitting the serving and avoiding evening doses deal with most of them. The real caution is interactions: if you take warfarin, insulin, cancer treatment or any regular medicine, speak to your GP or pharmacist before you start, and never change a prescribed medicine to take a supplement.
Vanessa holds a Level 3 Diploma in Nutrition and co-founded NutritionGo, where she leads editorial content. She researches and writes our guides, covering ingredient composition, traditional use and nutrition information, and fact-checks them against UK advertising rules and the GB Nutrition and Health Claims register. Every factual statement is verified against sources such as the NHS, the GB Nutrition and Health Claims register or peer-reviewed research on PubMed before publication. Vanessa is not a registered dietitian or healthcare professional, so this guide is for general information and is not a substitute for personal medical advice.
References
View all 3 references and sources
- National Center for Complementary and Integrative Health. Coenzyme Q10. Safety, side effects and interactions with warfarin, insulin and cancer treatment.
- Hathcock JN, Shao A. Risk assessment for coenzyme Q10 (ubiquinone). Regul Toxicol Pharmacol, 2006;45(3):282-8. Observed safe level of 1,200mg a day.
- Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. CoQ10 has no authorised health claims.